1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
Tandem-type image forming apparatuses having a transfer belt have become mainstream image forming apparatuses in recent years. In a tandem-type color image forming apparatus, image forming units provided for respective toner colors of cyan, magenta, yellow and black and each including an exposing device, a photoreceptor, a developer and the like are tandemly arranged in the transporting direction of a recording sheet (a medium on which an image is to be transferred).
In each image forming unit for a particular color, the exposing device forms a latent image on the photoreceptor, and the developer develops the latent image, thereby forming a toner image of the corresponding color. Subsequently, toner images of the colors are sequentially transferred and superposed one upon another onto a recording sheet conveyed by the transfer belt. In this manner, a full color toner image is formed.
Tandem-type image forming apparatuses may, however, cause color misregistration which is displacement of a transfer position of a toner image of each color due to displacement of an optical scanning position of each exposing device or the like, which results in a reduction in image quality. Accordingly, for tandem-type image forming apparatuses, it is necessary to detect the presence of color misregistration and correct the color misregistration.
In an image forming apparatus, detection and correction of color misregistration is performed using a toner mark pattern for a color misregistration measurement. The toner mark pattern is formed on a moving object, such as a recording sheet conveying belt, and read by a toner mark sensor (hereinafter, referred to as “TM sensor”). Based on the toner mark pattern read by the TM sensor, the amount of color misregistration is detected for each color, and an operation for correcting the color misregistration is performed according to the detected amount of the color misregistration.
The following has been proposed as technologies related to color misregistration correction using the above-mentioned toner mark pattern.
Patent Document 1 discloses a technology applied to an image forming apparatus for providing different types of writing error correction means in the space between two recording sheets. According to the disclosed technology, writing error correction can be performed without causing downtime.
Patent Document 2 discloses a technology applied to an image forming apparatus for correcting color misregistration by forming a toner mark pattern of an arbitrary number of toner colors, which is smaller than the maximum number of toner colors, in accordance with the space between two recording sheets.    [Patent Document 1] Japanese Laid-open Patent Application Publication No. 2005-289035    [Patent Document 2] Japanese Laid-open Patent Application Publication No. 2007-292936
The above conventional technologies correct color misregistration both in the main scanning direction and in the sub-scanning direction without changing the width of the space between the recording sheets and use a common toner mark pattern for the color misregistration correction. Accordingly, even in order to detect color misregistration only in the main scanning direction, it is necessary to calculate a distance between toner marks detected by a sensor. Note here that the term “toner mark” used in the description of the present application refers to a single mark (for example, an individual line) On the other hand, the term “toner mark pattern” refers to a collective set of one or more toner marks formed in one given space (e.g. a space between two recording sheets).